I think this was discussed a looong time ago, but I don't remember if there was any conclusive results, so, I bring it up again.
I just got two failing boards back from a customer. The CPU is the mega161-4AC.
On analysis, it turned out that both Flash memories were completely empty, all FF's.
The CPU's seemed otherwise okay, and after reprogramming, they worked nicely again.
No lockbits are set in the running systems, as they support firmware upgrade. The bootloader has a block, so it cannot reprogram itself.
The systems they're used in are pretty noisy, but the boards are power supplied from the USB port of a larger system. Also, a local regulator takes the voltage down to 3.3V, so power should be good. Anyway, no matter how much power drops there may be, the CPU should never loose it's Flash content.
This has been reported to Atmel, but I haven't got an answer yet.
In the meantime, has anyone else seen this happen ?